Silicon micromachining has been developed over the last decade as a means for accurately fabricating small structures. Such processing generally involves the selective etching of a silicon substrate and depositions of thin film layers of semiconductor materials. Silicon micromachining has been applied to the fabrication of micromachines that include a rotary or linear bearing that allows substantially unrestricted motion of a moving component in one degree of freedom. Such bearings have spawned the development of electrically-driven motors referred to herein as micromotors. Such micromotors have a planar geometry, gap separations on the order of 1-2 microns and lateral dimensions on the order of 100 microns or more.
Although such micromachines have been successfully fabricated and tested, many have not proved entirely satisfactory for the reason that difficulty has been experienced in constructed bearing arrangements. In short, such bearings have exhibited excessive friction and wear due to mechanical contact between stationary and moving parts.
In co-pending U.S. patent Application Ser. No. 07/810395, and assigned to the same Assignee as this application, the problem of bearing friction has been substantially alleviated by the provision of balanced torque stators, vertically disposed (and opposed) about a disk-like rotor that is tethered by a central bearing. The application of balanced, multi-phase signals to the opposed torque stators allows balanced electrostatic forces to be imposed upon the tethered rotor, which forces also act to impart rotative motion to the rotor.
In a parallel development to that described in the above-noted patent application, two of the inventors hereof also developed a levitation method that eliminates most mechanical contact between moving elements of a micromachine while, at the same time, enabling desired motion actions and/or forces. This invention is described in Cho et al. U.S. Pat. No. 5,015,906, issued May 14, 1991. The teachings of the Cho et al. patent are incorporated herein by reference. To provide a background for the invention to be described herein, a brief resume of the teachings found in the '906 patent will be presented. FIG. 1 hereof is a reproduction of FIG. 3 from that patent and achieves three-dimensional levitational stability of a freely movable planar structure 10. The embodiment of FIG. 1 includes a non-conductive substrate 12 from which two vertical walls 14 and 16 extend and support horizontal walls 18 and 20. Together, the walls form a cavity 22 with a narrow slit 24. Mounted on the under surface of walls 18 and 20 are conductive plates 26 and 28 are respectively. Similar conductive plates 26' and 28' are mounted on the upper surfaces of substrate 12, inside cavity 22 and directly below the upper disposed plates 26 and 28, respectively.
Planar structure 10 is emplaced in cavity 22 and includes a thin, non-conductive plate 30 sandwiched between conductive plates 32 and 34. A high frequency voltage source 36 is connected, on one side, to opposed plates 28 and 28' and on the other side, via inductors 38, to opposed conductive plates 26 and 26'. Planar structure 10 will levitate in stable equilibrium for all spatial orientations of the structure shown in FIG. 1 and substantially independent of gravity.
As is described in the '906 patent, the structure shown in FIG. 1 can be represented by an equivalent circuit including voltage source 36, inductors 38, the capacitance between plates 26, 26' and 28, 28', conductors 32 and 34, and the distributed resistance of the circuit. The equivalent circuit includes two parallel and opposed resonant circuits which, when properly energized, will cause the stable levitation of planar structure 10.
In operation, planar structure 10 is held in vertical equilibrium with a constant gap distance "d". Planar structure 10 is held in a stable position if the frequency of voltage source 36 has a frequency f.sub.s that is greater than the natural frequency f.sub.n of the parallel resonant circuits. The stability of levitation of structure 10 comes about as a result of the fact that the net forces acting on structure 10, when it is displaced from its equilibrium position, is restoring, i.e., an upward displacement of structure 10 produces a net downward force and vice versa.
A restoring force is achieved by having the frequency f.sub.s of voltage source 36 greater than the natural frequency of the afore-described resonant circuit. Thus, when structure 10 is perturbed from its equilibrium position, the capacitance values change which, in turn, change the voltages applied across the respective capacitances. As a result, the value of the electric force field is altered within cavity 22. The frequency f.sub.s is such that the rate of increase (or decrease) in the oscillating electric force field is greater than the rate of decrease (or increase) in the gap distance "d". Therefore, structure 10 experiences a net vertical restoring force that is essentially a null at some levitation position located between the upper and lower levitating plates respectively.
A horizontal, stable equilibrium is also imparted to structure 10 when it is centrally positioned in cavity 22. From this central position, a horizontal displacement of structure 10 (e.g., to the right or the left), will produce a net horizontal restoring force on structure 10 in the direction opposite to the direction of displacement. For instance, if structure 10 is moved slightly to the right from the position shown in FIG. 1, the attractive forces produced by plates 26, 26' and 28, 28' will have a net horizontal component to the left and pull structure 10 back to the center. The horizontal component of the attractive force on structure 10 is essentially zero when it is symmetrically disposed between plates 26 26' and 28, 28'.
FIG. 2 is a reproduction of FIG. 6b of the '906 patent. It shows the application of the levitational principles described above to a rotating embodiment. In FIG. 2, a disk-shaped levitating rotor 50 has a serrated edge and a plurality of radial conductive members 52 disposed on its surface. A drum shaped housing 54 (shown only in plan view) has a plurality of radially disposed electrodes 56 positioned over rotor 50. Groups of three of radial electrodes 56 are connected in a three phase relationship to a voltage source/inductor arrangement identical to that described for FIG. 1. Each of the voltage sources, in accordance with the above-described principle have frequencies of oscillation greater than the natural frequencies of the resonant circuits connected thereto. The circuits enable the levitation of rotor 50 through the attractive forces induced between conductive strips 52 and 54. Conductive strips 52 are arranged radially about rotor 50 and have an asymmetrical spacing with respect to electrodes 56. Thus, when the voltage sources are sequentially energized, radial forces are induced in rotor 50, thereby enabling its rotation.
Further details concerning the above-described invention can be found in "A Proposal for Electrically Levitating Micromotors", Kumar et al., Sensors and Actuators, Vol. 24, 1990, pages 141-149 and "Electric Levitation Bearings For Micromotors", Kumar et al., Digest of Technical Papers, IEEE International Conference on Solid State Sensors and Actuators, pp. 882-885, 1991.
As is shown by the above described prior art, it is possible, using an RF circuit, to levitate a conducting rotor in a micromotor design. The levitation forces can be applied in three dimensions to suspend the rotor against the force of gravity and permit a design that does not include a friction bearing. However, the switching technique employed to impart rotational movement to the levitating structure shown in the '906 patent, while satisfactory, presents circuit design complications that are somewhat expensive to implement.
Accordingly, it is an object of this invention to provide an improved vertical-drive, levitated micromotor.
It is another object of this invention to provide an improved vertical-drive, levitated micromotor that exhibits simplified circuit requirements.
It is still another object of this invention to provide a levitated micromotor arrangement that provides improved balance and levitation characteristics for a non-tethered rotor.